FIG. 1 is a diagram of a prior art current sensing circuit in which the current to be sensed, designated by Is, flows across a resistor Rs (defining a voltage Vi) while a common-mode voltage Vcm is applied to a terminal of the resistor Rs opposite a terminal where the current Is to be measured flows. The voltage sensed across the resistor Rs is then amplified by an operational amplifier 1 having a differential gain defined by Gd. The gain of the amplifier should, in principle, be constant to precisely amplify the sensed voltage. In some cases, the gain might be used to adjust the amplification as required.
The output voltage of the operational amplifier 1, designated as Vo, is obtained by multiplying the input voltage Vi by the differential gain Gd. There are basic requirements for the circuit shown in FIG. 1. First, a high impedance at the terminals of the resistor Rs is required, and therefore across the input of the operational amplifier 1. Another requirement is for the differential gain Gd to be precise and insensitive to operating conditions. An output voltage is required to be independent of the common-mode voltage Vcm, i.e., a high common-mode rejection ratio (CMRR) is desirable. Other requirements include a wide linear input range, and a minimal consumption of the silicon surface area needed for forming integrated circuits incorporating the circuit.
FIG. 2 is a view of a first embodiment of the circuit shown in FIG. 1, wherein the terminals A and B designate input points for the operational amplifier 2 for the voltage across the terminals of the resistor Rs, as shown in a similar way in FIG. 1. FIG. 3 is a view of a second embodiment of the circuit shown in FIG. 1. In both of these figures, the amplification gain is determined by ratios between resistors. Referring to FIG. 2, there are four resistors in the circuit for determining the amplification gain, whereas in FIG. 3, there are six resistors in the circuit for determining the amplification gain. Furthermore, a single operational amplifier 2 is used in the circuit of FIG. 2, while three operational amplifiers 3, 4, 5 are used in the circuit of FIG. 3.
Since the gains of the circuits of FIGS. 2 and 3 are determined by the ratios between the resistors, the resistors must be as equal as possible to each other to have accurate amplification. The equalization of the resistors is also necessary to avoid influencing the offset and the common-mode rejection ratio. Furthermore, the input impedance of the circuit of FIG. 2 is inherently low. Referring to the circuit of FIG. 3, a significant disadvantage is that it is more expensive to manufacture because of the number of operational amplifiers required. Therefore, the silicon surface area needed for forming the circuit on an integrated circuit is not negligible.